When the diaphragm and external intercostal muscles contract, you inhale. These two muscle groups are the primary drivers of inspiration, working together to expand the chest cavity, drop the pressure inside your lungs below atmospheric pressure, and pull air in. Every quiet breath you take starts with this coordinated contraction.
How These Muscles Create a Breath
The diaphragm is a dome-shaped muscle that sits beneath your lungs, separating your chest from your abdomen. When it contracts, it flattens and pulls downward, increasing the vertical space inside the chest cavity. During a normal, relaxed breath, the diaphragm descends about 1 to 2 centimeters. During a deep breath, it can drop 7 to 11 centimeters.
At the same time, the external intercostal muscles (the muscles between your ribs) contract and pull the ribs upward and outward, expanding the chest cavity in a lateral direction. Together, these two movements increase the total volume of the thoracic cavity in all dimensions: taller from the diaphragm descending, wider from the ribs swinging out.
Why Air Flows In
The reason air enters the lungs comes down to basic physics. Pressure and volume have an inverse relationship: when you increase the volume of a sealed space, the pressure inside drops. This principle, known as Boyle’s law, is the engine behind every breath you take.
As the chest cavity expands during contraction, the lungs expand with it because they’re attached to the chest wall by a thin layer of fluid. This expansion drops the pressure inside the tiny air sacs of the lungs to about negative 1 centimeter of water pressure, just slightly below atmospheric pressure. That small difference is enough to create a pressure gradient, and air rushes in through your nose or mouth to equalize, much like a vacuum. Meanwhile, the pressure in the space between the lungs and chest wall (the intrapleural space) drops from its resting value of about negative 4 mmHg to an even more negative value, helping keep the lungs inflated and expanding.
A single quiet breath pulls in roughly 500 mL of air in an average adult male and about 400 mL in an average female, roughly 7 mL per kilogram of body weight. Of that, about 350 mL actually reaches the gas-exchange surfaces deep in the lungs. The remaining 150 mL stays in the airways (nose, throat, and large bronchial tubes) where no oxygen exchange occurs.
How the Brain Controls the Process
You don’t have to think about breathing because the brainstem handles it automatically. A cluster of neurons in the brainstem monitors carbon dioxide levels in your blood. When CO2 rises, these neurons send signals down two pathways simultaneously.
The phrenic nerve, originating from spinal nerve roots C3 through C5 in the neck, carries the signal to the diaphragm. The intercostal nerves, branching from the thoracic spine, activate the external intercostal muscles. Because the phrenic nerve originates relatively high in the spinal cord, people with certain spinal cord injuries below C5 can still breathe on their own, even if they’ve lost movement in their limbs.
What Happens When You Exhale
Normal, quiet exhalation is essentially the reverse, and it’s passive. The diaphragm and external intercostals simply relax. The chest cavity shrinks back to its resting size, the elastic tissue of the lungs naturally recoils (like a stretched rubber band returning to shape), and the pressure inside the lungs rises above atmospheric pressure. Air flows out.
No muscular effort is needed for a relaxed exhale. This is why breathing at rest feels effortless: only the inhalation phase requires active muscle contraction.
Accessory Muscles During Heavy Breathing
During exercise, illness, or any situation that demands more air, the diaphragm and external intercostals get help. Several accessory muscles kick in to pull even more air into the lungs. The sternocleidomastoid muscles in the neck, the scalene muscles along the sides of the neck, and the pectoralis muscles in the chest all assist by further lifting the ribcage and upper sternum.
Forceful exhalation also becomes active during heavy breathing. The abdominal muscles contract to push the diaphragm upward, and the internal intercostal muscles pull the ribs downward and inward, compressing the chest cavity more aggressively than passive recoil alone. This is why you can feel your abs working when you’re breathing hard after a sprint.
In healthy people, these accessory muscles are only recruited when metabolic demand increases. If someone uses them during rest, visible as neck muscles straining or nostrils flaring with each breath, it’s typically a sign of respiratory distress.